(1) Field of the Invention
The invention is related to a landing gear vibration absorber with the features of claim 1 and to a method of operating said landing gear vibration absorber with the features of claim 10.
(2) Description of Related Art
It is known to attenuate or absorb the vibration of a rotorcraft by means of active or passive attenuator or absorber devices; passive devices are resonators tuned to a determined frequency, while active devices rely on an actuator that is controlled so as to deliver forces that oppose measured vibration.
The document U.S. Pat. No. 7,461,729 A discloses a device for suspending a battery of an aircraft capable of providing a vibration attenuator system. The device includes at least one spring and further includes an adjustment element enabling the stiffness of the spring to be adjusted. Typical battery weights are in the range of 20 to 40 kg. This large mass cannot be placed everywhere in the helicopter.
The document U.S. Pat. No. 4,311,213 A describes a mechanism suitable for filtering the vibration to which a rotorcraft is subjected, and in particular for filtering the vibration to a seat fitted into the rotorcraft. An array of link members is interposed between the rotorcraft fuselage and a suspended seat-carrying platform. The link members are oriented substantially in crossed directions, and with hinge engagement on the fuselage, serving to carry the platform in order to suspend it. Two link members are connected to each other by a torsion shaft or arm. Each link member comprises a deformable member that is interposed between the platform and the fuselage and a lever arm for operating the deformable member carrying a weight. The weight is carried by the lever arm at its free end opposite from its end anchored to the fuselage. The torsion shaft engages hinge bearings via which the lever arms of the corresponding link members are hinged to the fuselage at their anchor ends. Although that mechanism provides effective filtering, it is complex, expensive, bulky, and requires a specific arrangement and organization of the array of link members depending on the implantation. The arrangement of the array of link members and the ways in which they are interposed, need to be determined specifically for each individual application.
The document U.S. Pat. No. 4,088,042 A describes a vibration isolating system that is compact, of nodal type, filtering multiple frequencies and fitted to a helicopter. This system forms a four-branched cross, with four dampers at the ends of the branches.
The document US 2011/0095132 A provides a vibration filter mechanism for aircraft equipment. A weighted lever arm is hinged via bearings associated respectively with a first structure connected to a fuselage and with a second structure connected to the equipment. Deformable means oppose pivoting movement of the lever arm. The lever arm is arranged as a one-piece fork that comprises a pair of branches that are interconnected by a crossbar and that are hinged to the bearings about spaced-apart parallel pivot axes (A1, A2). The fork carries a torsion shaft that extends between the branches at their free ends, the torsion shaft constituting the weight weighting the lever arm and the deformable means of the mechanism.
The document U.S. Pat. No. 4,172,570 A discloses helicopter landing gears suspended by a tuning spring from the helicopter airframe to provide limited relative movement between the landing gear masses and the airframe. As the aircraft vibrates due to vertical or in-plane forces at the rotor head, the landing gear masses react oppositely thereto, thus generating balancing forces which are applied to the airframe to balance the rotor excitations.
The document U.S. Pat. No. 5,620,068 A discloses an actively-controlled resonant-type force generator adapted to be attached to a structure with a mass mounted for movement relative to the structure and a plurality of springs operatively arranged between the mass and the structure. A servoactuator is arranged to controllably excite the mass-spring system. The actual force (Fa) transmitted from the mass to the structure is compared with a commanded force (Fc) to produce a force error signal (Fe). The actuator is caused to produce a velocity as a function of the error signal. The gain of the closed force loop is selected so that the resonance of the mass-spring system has an effective damping ratio (zeta) greater than about 0.5, and preferably about 0.7. Thus, the mass-spring system will not be substantially resonantly excited by vibrations of the structure near its resonant frequency (omega n).
The document GB 1205263 A discloses an aircraft with a ski type landing gear which comprises two main skid members, at least two forward cantilever members and an arcuate rear cross-member. The cantilever members are built into the skid members and pivotally attached to a fuselage. The rear cross-member, also built in to the skid members, is attached to the fuselage and has vibration and/or recoil dampers attached near the lower ends of the cross member and incorporating self-aligning ball joints at both the cross-member and fuselage attachment points. Attachment lugs are provided on the skid members for ground handling wheels and drag struts from the aircraft to the rear cross-member may be fitted for yaw stiffness.
The document U.S. Pat. No. 3,716,208 A discloses a landing gear for helicopters in which a member having a static spring rate and plastic yielding characteristics is mounted in series with a member having a static spring rate and a velocity-sensitive restraining force. The combination serves to provide for appropriately “soft” landings under conditions of light gross weight and/or low vertical descent speed, and is also capable of absorbing the forces generated during “hard” or crash landings under conditions of heavy gross weight and/or high vertical descent speed. By designing the yield point of the plastic yielding member to a force approximately equal to the resistance offered by the velocity-sensitive unit at the time that is “bottoms out,” the efficient energy absorbing properties of the plastically yielding member are employed to extend the energy absorbing capabilities of the gear.
Fleet statistics, e.g. for EC135 from nearly 1000 helicopters show a large scatter of cabin vibration levels with an obvious influence of the landing gear configuration of the particular helicopter, especially if the eigenfrequencies of the landing gear are within the range of the helicopters main excitation frequencies.